"Blue Valentine" director Derek Cianfrance can't quite thread the needle between John Cassavetes and David Lean, but it's fun to watch him try.

Several miles off the coast of Tasmania, across a stretch of water so cold and jagged that it makes the river Styx seem like a kiddie pool, a tight-lipped World War I vet lives on a tiny island by his lonesome. Unlike most of the lighthouse keepers who come to the impossibly remote shores of Janus Rock, Tom Sherbourne (Michael Fassbender) shows up solo — all the better for someone who wants to “Get away from things for a while.” Eventually, Tom is joined by Isabel (Alicia Vikander), the bright-eyed local beauty who agrees to be his wife. The idyllic solitude they share together might last forever if not for the series of devastating miscarriages that sink them like a squall, but the sea is full of hope for those who keep a desperate eye on the shore.

As a Decemberists song, “The Light Between Oceans” could have been a masterpiece. As a middlebrow period melodrama in the vein of “The Painted Veil, “Lust, Caution,” and other swooning romances scored by the great Alexandre Desplat, the film — much like the twinkling musical tempest the French composer wrote for it — is lush and lacking in equal measure.

Based on M.L. Stedman’s 2012 novel of the same name and dutifully adapted to the screen by “Blue Valentine” director Derek Cianfrance, “The Light Between Oceans” finds the stiflingly male auteur ditching the bruised heart bravado of his previous films, tempering the go-for-broke intensity he brought to “The Place Beyond the Pines” with a more classical sense of composition. Don’t worry, he still serves up an endless buffet of unhinged emotions and extreme close-ups (every one of Vikander’s individual tears has enough screen time to earn its own SAG card), but working from someone else’s material seems to have reined him in a bit.

In the past, Cianfrance burdened his characters with pounds of affect and sent them shuffling into the sea. Here, locked into a story that is almost constantly on the verge of overplaying its hand, he manages to sublimate his maximalist flair into the film’s otherworldly environments, allowing Tom and Isabel to react naturally to become the hapless victims of the plot they set into motion.

And what a plot it is. A lucid and involving tear-jerker that only tumbles into excess during its woefully misguided final minutes, “The Light Between Oceans” may lack the poetry required to push it over the top, but it swells with tremendous heartache all the same. It’s a classic moral dilemma, a sign from God in the form of a felony: Just when Tom and Isabel have lost all hope of having a child, a helpless baby girl floats towards Janus in a dinghy. She’s alone save for the fresh corpse — presumably her father — who lies in the boat beside her, and a small rattle in the shape of an owl (a keepsake that inevitably becomes a clue).

Tom insists that they report their discovery to the mainland, but Isabel — tears in her eyes — begs her husband to let them raise the cooing bundle of blonde on the island. They know that, somewhere on the continent, a mother has lost her daughter. They know that their happiness might come at the expense of an innocent woman’s, and perhaps also that of her family. They call the girl Lucy.

The years pass, the infant grows into an absurdly cute four-year-old (Florence Clery), and the guilt gnaws at Tom’s heart. Like the Roman god for whom his adopted home has been named, Tom finds himself staring in two separate directions (though always with the same clenched face), one eye on the sins of his past and another on the hopes of his future. His job is to help guide ships as they pass from one ocean to another, and this deeply tortured man is stuck in the middle, a crossing guard for the happiness that he never gets to have for himself. It’s only a matter of time before he causes a collision.

Fassbender could deliver the requisite intensity with both eyes closed, and it’s a joy to watch him thaw during the relatively warm second act, but Tom only sinks into himself as the story around him grows frenzied. Adam Arkapaw’s raw and resplendent cinematography expresses the impassive character’s inner turmoil more vividly than Fassbender is allowed to, and the film’s Plutonian sunsets soon become more evocative (and interesting) than the characters upon whom they cast their periwinkle light. It’s telling that Cianfrance withholds a full view of Janus until halfway through the movie at the moment of Tom’s greatest emotional isolation — every man is an island, and some find that living by themselves is easier than living with themselves.

Isabel, by contrast, is a raw nerve, a Thomas Hardy heroine married to a John Steinbeck hero. Vikander quivers with the best of them, her deep brown eyes making it easy to empathize with the desperation that underscores her defining act of selfishness. And Rachel Weisz, who enters the fray halfway through as Lucy’s birth mother, continues her string of exquisitely restrained supporting roles. But Cianfrance, an unsparing Artist (with a capital “A”) who has made a name for himself by flushing the truth out of his actors like a cold sweat, shows an uncharacteristic willingness to let this cast off the hook. The film’s climactic scenes arrive in a flurry of life-changing reversals and last-minute declarations, but each of them cheats viewers out of the payoffs that have been promised us by the movie’s impossible twists of fate. A tacked-on coda only serves to underline the missed opportunities of a movie in which the most dramatic moments are all dissolved into the scenery.

Cianfrance has described his latest film as “A John Cassavetes movie in a David Lean landscape,” but — contrary to what the director’s previous work might lead you to expect — “The Light Between Oceans” leans too far towards the latter. But this is a widescreen ode to the beauty of absolution, told with such constant sincerity that you can’t help but want to forgive its flaws. “You’re still a mother or father even if you no longer have a child,” Isabel declares in the first act, oblivious to the fact that those words will come back to haunt her in the third. But Cianfrance never forgets that they could be a comfort as well, if only a cold one.

Grade: B

“The Light Between Oceans” opens in theaters on Friday, September 2nd.